1. Field of the Invention
This invention relates generally to semiconductor packaging, and more particularly, to a package of a semiconductor device having increased ability to limit moisture flow through the packaging material and into the area of the die and die pad
2. Discussion of the Related Art
Reference is made to FIGS. 1-3, wherein a typical packaged semiconductor structure 10 is shown. As is well known, a die pad 12 has a die 14 epoxied thereon and tie bars 16, 18 extending therefrom. Bonding wires 20 connect the die 14 to the inner ends of leads 22. The die pad 12, die 14, wire bonding 20 and inner ends of the leads 22 are packaged in epoxy 24, commonly referred to as plastic packaging, with the outer ends of the leads 22 extending from the epoxy 24. These leads 22 may be plugged into a printed circuit board 26, as shown in FIG. 3.
Over a period of time, moisture 28 diffuses through the plastic package 24 and into the area of the die 14, die pad 12, and wire bonding 20. Through the operation of the device 10, heat buildup can cause moisture in those critical areas to vaporize, creating large amounts of steam pressure in pockets adjacent to these critical areas. In the case of moisture near the lower side of the die pad 12, blisters may form of sufficiently large size to cause connection problems between the printed circuit board 26 and the device 10. In the case of moisture near the upper side of the die 14, wire bond failure, die face damage, and/or die fracture can occur. Obviously, situations where such delaminations can occur are highly undesirable and can result in severe reliability problems.
Current practice is to measure how well each packaged device performs under JEDEC stress testing (specified temperature and humidity environment for a specified time for each Level). Manufacturers of semiconductor devices strive to reach Level 1 as that is the only level for which neither device baking nor a hermetic bagging are required. This xe2x80x9cbake and bagxe2x80x9dprocess involves additional expense, and would not be necessary if the package were inherently resistant to moisture flow therethrough to the area of the die 14, die pad 12 and wire bonding 20.
FIG. 4 shows a packaged semiconductor device 29 which utilizes an anodized aluminum heat spreader 30 in contact with the die pad 32 for dissipation of heat buildup during the operation of the device 29. In the fabrication of the device 29, the heat spreader 30 is placed in the well of a transfer molding machine, legs 34 extending from the heat spreader 30 contacting the bottom of the well, and a lead frame 36 having a die 38 associated therewith and wire bonded thereto is placed over the heat spreader 30 with the die pad 32 in contact with the heat spreader 30, so as to be an interference fit with the heat spreader 30. As molding compound 40 is forced into the area of the die 38 and die pad 32, air can be trapped above the heat spreader 30 and beneath the die pad 32 to form an air void therebetween. The heat spreader 30 includes apertures 42 therethrough to allow this air void to pass through the apertures 42 and away from the area of the die pad 32, thereby relieving pressure from this area. While this configuration is effective for this purpose, the problem of limiting moisture ingress into the area of the die pad 32, die 38, and wire bonding is not addressed. Indeed, the apertures 42 in the heat spreader 30 will readily allow moisture therethrough directly into the area of the die pad 32 in contact with the heat spreader 30. Furthermore, the problem of moisture ingress into the area of the die 38 and wire bonding through the packaging material thereabove is not addressed.
Therefore, it would be highly desirable to provide a system for limiting moisture diffusion through the plastic packaging material to the critical areas of the die, die pad, and wire bonding, so that the delamination problems described above are avoided.
In the present invention, in the environment of a semiconductor die epoxied to a die pad, with tie bars extending from the die pad, barrier structures are secured to the tie bars so as to have barrier bodies positioned on opposite sides of and overlying the die-die pad assembly. Each barrier body is spaced from the die-die pad assembly, and has a plurality of small apertures therethrough. As moisture diffuses through the plastic package material, the barrier bodies act as baffles to limit such diffusion in the direction of the die-die pad assembly to only that passing through the apertures. From these apertures the moisture diffuses in a spreading manner. Additional barrier structures may be provided which have barrier bodies overlying the above-described barrier bodies, the additional barrier bodies having apertures therethrough which are non-aligned with the apertures of the first-described barrier bodies to provide an even longer moisture flow path to the die-die pad area.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there are shown and described embodiments of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.